ASAR WIDE-SWATH SINGLE-LOOK COMPLEX PRODUCTS: PROCESSING AND EXPLOITATION POTENTIAL Ralph Cordey (1), Tim Pearson (2), Yves-Louis Desnos (3), Betlem Rosich-Tell (3) (1) European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands. Email: ralph.cordey@esa.int (2) RSAC Ltd., support to ESRIN, Via Galileo Galilei, 00044 Frascati, Italy. Email: tim.pearson@esa.int (3) European Space Agency, ESRIN, Via Galileo Galilei, 00044 Frascati, Italy. ABSTRACT/RESUME The European Space Agency (ESA) currently supports interferometry and other applications of single-look complex data from the Envisat ASAR through the provision of single-polarised and alternating-polarisation narrow-swath SLC products and SLC wave-mode imagettes. Wide-swath SLC products have not yet been offered to users on an operational basis. ESA has sponsored studies into the generation of single-look complex products from ASAR in its wide-swath mode, and has procured a prototype processor. It now seeks to implement a wide-swath SLC product operationally and act to encourage the widespread exploitation of such data. In support of this implementation, an ad-hoc working group of scientists has been formed to help examine and promote the likely applications of wide-swath SLC data. Significant potential for the exploitation of wide-swath SLC products is believed to exist in a number of user communities. These include those wishing to use large-area INSAR techniques in seismology and other surface movement applications, and in the larger-scale characterisation of atmospheric water vapour. Non-INSAR applications are also expected to have strong interest, especially for oceanography. A wide-swath SLC product may offer the potential to estimate a component of ocean surface currents over a 400km-wide swath and it may similarly extend coverage of contiguous directional wave-spectra. 1 INTRODUCTION The Wide-Swath Mode of Envisat s ASAR instrument offers considerable benefits to both scientific and applications users through a combination of its wide coverage and comparatively rapid revisit capabilities. The role of such a mode is well-established for sea-ice mapping and monitoring, and we note increasing interest in its use for monitoring land surface processes with short timescales. To date, the only operational Level-1B data product offered by ESA from the ASAR Wide-Swath Mode has been a multi-look detected product (ref: ASA_WSM_1P), intended to support applications that exploit intensity data. No processed image product retains phase information or can support the analysis of separate radar looks. The addition of a single-look complex product is considered timely to support the future development of a number of applications of Wide-Swath Mode. These may be expected to include INSAR applications based either on wideswath/wide-swath pairs or wide-swath/image mode pairs, applications of ocean current mapping, large-area ocean wave retrievals, and atmospheric water vapour characterisation. Here we report on activities supporting the implementation of a new operational ASAR product, ASA_WSS_1P, retaining phase information and allowing access to individual radar bursts. Those include the implementation of a prototype Wide-Swath SLC processor, the production of sample SLC datasets and the creation of an ad-hoc working group to foster understanding and promotion of applications of the operational product. It is intended that products will be available for order by Envisat users at the end of 2004. 2 SUMMARY OF ENVISAT ASAR WIDESWATH CHARACTERISTICS Details of the operation and data products from Envisat s C-band ASAR can be found in [1]. In its Wide-Swath Mode, the instrument illuminates in succession five sub-swaths, designated SS1 to 5, that provide a total ground-range Proc. of FRINGE 2003 Workshop, Frascati, Italy, 1 5 December 2003 (ESA SP-550, June 2004) 28_cordey
coverage of approximately 406 km (Fig. 1). The polarisation of the signals used in this mode may be selected to be either HH or VV. 44 16 ~ 0.2s Sub -swaths SS1 SS2 406km SS3 SS4 SS5 Fig. 1. Sketch of burst imaging concept for Envisat ASAR Wide Swath Mode. Bursts of pulses are directed to the various sub-swaths in succession. The diagram does not seek to represent the order in which sub-swaths are illuminated or the instantaneous area of illumination. The Wide-Swath Mode is a burst mode of radar operation. The instrument illuminates a given sub-swath with a limited number of pulses before moving to the next sub-swath in the sequence. This contrasts with the continuous illumination of a swath in ASAR s Image Mode. Thus, while in Image Mode, a given target will produce radar returns covering the full range of Doppler frequencies as the ASAR beam passes over it, in Wide-Swath Mode a target will only produce returns with Doppler frequencies corresponding to its range of geometries relative to the satellite during the burst period. A key issue for applications of Wide-Swath complex data products is that ASAR generates more than one cycle of pulse bursts during the period equivalent to the synthetic-aperture formation in conventional Image Mode. Thus, a given target within a sub-swath will be illuminated by multiple bursts of pulses while it remains accessible within a radar beam. The interval between those bursts of pulses is approximately 0.19 seconds. The pulse bandwidths, pulse timing sequences and processor settings in ASAR Wide-Swath Mode have been subject to optimisation since the launch of Envisat. Details can be found in [2]. 3 WIDESWATH SLC PROCESSING AND ISSUES FOR OPERATIONAL PRODUCT GENERATION A prototype wideswath SLC processor for ASAR has been created by the Politecnico di Milano and the Politecnico di Bari under contract to ESA [3]. This processor has been used to set requirements for an operational implementation in the ESA integrated processor, PF-ASAR, and to generate specimen products for applications demonstrations. The key features of the planned product, ASA_WSS_1P, are given in Table 1. The product will be non-stripline (i.e. it will be processed into scenes on demand) and there will be no correction of residual Doppler frequencies. Such noncorrection is intended to support the interpretation of residual Doppler frequencies for ocean surface current estimation. Antenna pattern correction will be supported to assist intensity-based wind-wave processing algorithms. The delivery medium will be DVD. Given the current operational limitation on product data volume of 2GByte, a significant issue for the product is the trade-off between product coverage and sampling rate in azimuth. The possible applications requirement to track
residual Doppler frequencies implies a higher spatial sampling rate than that formally required to sample the local data bandwidth. It is undesirable to offer a wide swath product that does not exploit fully the potential area coverage of that instrument mode and so alternative approaches to providing a capability for unambiguous Doppler frequency tracking at an acceptable azimuth sampling are currently being explored by ESA. Data representation Coordinates Product size Product length Doppler accuracy Phase quality Table 1. Summary parameters for Wide-Swath SLC data product Complex, 16 bit I, 16 bit Q Slant range, zero-doppler coordinates <2GByte TBD <25Hz <10 rms, <0.5 mean, with phase continuity between bursts and between sub-swaths 4 INITIAL PRODUCT EXAMPLES The prototype wide-swath SLC processor has been used to generate a set of ASAR products intended to illustrate a range of applications. The scenes selected are listed in Table 2 and located in Fig. 2. Fig. 3 shows a detected version of an output sub-swath illustrating the individual processed bursts and the imaging of targets on the Earth in multiple bursts. Table 2. Initial specimen wide-swath SLC products Location Orbit/Track Date Galicia 3741/180 17 th November 2002 Cape Hatteras 4903/340 6 th February 2003 Spain 1 4249/187 22 nd December 2002 Spain 2 6754/187 15 th June 2003 Fig. 2. Locations of initial specimen products 5 WS-SLC WORKING GROUP AND EARLY RESULTS ESA has formed an ad-hoc working group of interested scientists and applications developers in order to prepare effectively for the release of operational wide-swath SLC products. That preparation entails the provision of advice on
sub swath 4 full sub-swath width one burst multiple imaging of all points Fig. 3. Detected representation of a single sub-swath. Azimuth is vertical and successive bursts show spatial overlap. issues relating to the operational implementation of the wide-swath SLC processor, the assessment of the science and applications potential of the new product, and assistance in the provision of information to other potential users. Membership of the ad-hoc group is not closed and new members are welcome to access and report back on the existing datasets. Proposals for additional datasets to be processed by the prototype processor are welcome also. In the first instance, interested parties should contact the authors of this paper. ESA is pleased to identify early results of the activities of the ad-hoc working group. Fig. 4 shows results of initial processing to retrieve contiguous large-area ocean wave information from the Galicia scene. The time lag between consecutive bursts of 0.19s appears to be sufficient to create a useable phase change in the image spectrum for unambiguous wave direction retrieval while maintaining correlation at the scale of dominant wave field components. 71% of the retrievals gave unambiguous wave propagation directions, which was sufficient for continuity considerations across the scene to permit complete ambiguity removal. An attempt to demonstrate wide-swath / wide-swath interferogram generation from the pair of images acquired over Spain was unsuccessful. However, a first successful interferogram has been generated by Politecnico di Milano and a visualisation of the interferogram is presented in Fig. 5. This has been created from a pair of VV-polarised wide-swath mode images acquired over the Low Countries on 1 st December 2002 (orbit/track 3941/380) and 5 th January 2003 (4442/380 -a temporal separation of 35 days). Successful interferogram generation from wide-swath pairs introduces an additional data selection criterion in addition to the familiar issues of temporal and baseline coherence faced by conventional strip-map interferometry. That is the need for overlap in the Doppler spectra of the individual bursts that illuminate a given point on the ground. This equates to a requirement for a level of timing synchronisation between the bursts of pulses in the two data acquisitions. At present, no attempt at synchronisation is made between different data takes in wide-swath mode by ASAR, but investigations are being undertaken by ESA to examine possible improvements to this situation. A tool would clearly be useful to users in order to screen archive datasets for possible synchronisation, and the provision of such a tool is also the subject of ongoing study.
Fig 4. Retrieved contiguous ocean wave field information based on the wide-swath Galicia scene. Images courtesy of B. Chapron (IFREMER) Fig. 5. The first successful wide-swath / wide-swath interferogram from Envisat ASAR. The location is the Low Countries of NW Europe. (Image courtesy of A. Monti-Guarnieri, Politecnico di Milano) ACKNOWLEDGEMENTS We thank the initial members of the WS-SLC Ad-Hoc Working Group for their contributions to this work. They are: Dr. Bertrand Chapron (IFREMER); Dr. Alberto Moreira & Dr. Johannes Schulz-Stellenfleth (DLR); Dr. Ramon
Hanssen (Technical University of Delft); Prof. Johnny Johannessen (NERSC); Dr. Harald Johnsen (NORUT); and Prof. Andrea Monti-Guarnieri (Politecnico di Milano). The prototype wideswath SLC processor used in this work was produced under ESA contract by Politecnico di Milano and Politecnico di Bari, contract number 15609/01/NL/SF. REFERENCES [1] European Space Agency, Envisat ASAR Product Handbook, Issue 1.1, December 2002 (on line availability via http://envisat.esa.int/dataproducts/) [2] Rosich, B., ASAR Image and Wide Swath Mode Optimisations and Product Quality Update, Proc. Envisat Validation Workshop, 9-13 December 2002 (http://envisat.esa.int/pub/esa_doc/envisat_val_1202/proceedings/) [3] Cafforio, C., Guccione, P. & Monti-Guarnieri, A., Envisat WS Complex Product Requirement Analysis, Politecnico di Milano Report for ESA Contract 15609/01/NL/SF, 2003.